MECHANICS OF THE CIRCULATION IN THE VESSELS 131 



it undergoes a great and sudden increase. A part of this increase 

 is to be attributed to the arterioles, which, although individually 

 very narrow, have a total bed considerably greater than that of the 

 arteries from which they spring. Where the arterioles pass into 

 the capillaries proper, a further and a still greater and more abrupt 

 increase in the bed occurs. At the venous end of this region the 

 cross-section is again somewhat abruptly contracted, and then 

 gradually lessens as the right side of the heart is approached; but 

 the united sectional area of the large thoracic veins is greater than 

 that of the aorta. 



Attempts have been made to measure the blood-pressure in the 

 capillaries by weighting a small plate of glass laid on the back of one of 

 the fingers behind the nail, until the capillaries are just emptied as 

 shown by the paling of the skin (v. Kries), or by observing the height of 

 a column of liquid that 

 just stops the circula- 

 tion in a transparent 

 part (Roy and Graham 

 Brown). The last- 

 named observers found 

 that a pressure of 100 

 to 150 mm. of water 

 (about 7 to ii mm. 

 of Hg) was needed to 

 bring the blood to a 

 standstill in the capil- 

 laries and veins of the 

 frog's web; that is, 

 about a third of the 

 blood-pressure in the 

 frog's aorta. The pres- 

 sure in the capillaries 

 at the root of the nail 



Fig- 59- Relation of Blood- Pressure, Velocity, and 

 Cross-Section. The curves P, V, and S represent the 

 blood-pressure, velocity of the blood, and total cross- 

 section respectively in the arteries A, capillaries C, 

 and veins V. 



in man varies from 



30 to 50 mm. of mercury, as estimated by the method of v. Kries. But 

 the method is exposed to serious errors. The method of measuring the 

 venous pressure described on p. 132 can also be applied to the capillaries, 

 and is somewhat more satisfactory. 



Under certain conditions the pulse-wave may pass into the 

 capillaries and appear beyond them as a venous pulse. Thus, we 

 shall see that when the small arteries of the submaxillary gland are 

 widened, and the vascular resistance lessened, by the stimulation of 

 the chorda tympani nerve, the pulse passes through to the veins. 

 And, normally, a pulse may be seen in the wide capillaries of the 

 nail-bed especially when they are partially emptied by pressure 

 as a flicker of pink that comes and goes with every beat of the heart. 



We have seen that the lateral pressure at any point of a uniform 

 rigid tube through which water is flowing is proportional to the amount 

 of resistance in the portion of the tube between this point and the outlet. 

 In any system of tubes the sum of the potential and kinetic energy 

 must diminish in the direction of the flow; and although the problem 



